。
Reading a good book is always worth it.
Literal
Good-book [object-を] read-if, without-fail that-much [of-の] effect [subject-が] exists.
~ば必ず ('if X, then without fail Y') is a conditional-strong consequence pattern. 良書 ('good book') — the modifier 良 ('good') + 書 ('book'). それだけの効果 ('that much benefit') — the pronominal それ refers back to the conditional action.